(1) Field of the Invention
The present invention relates to material properties measurement and, more particularly, to a method for measuring material properties using wall displacement measurements recorded during an insertion loss experiment.
(2) Description of the Prior Art
Insertion loss is a common measurement that is used to determine how effective a piece of material attenuates acoustic energy at a specific frequency. Insertion loss is calculated by projecting acoustic energy at piece of material and measuring the pressure on the projector side and the opposite side of the material, normally with hydrophones.
FIG. 1 depicts a typical setup for insertion loss. Sound pressure is transmitted to a test sample 10 by an acoustic projector 12. Acoustic projector 12 can transmit an acoustic wave at a preset frequency. Using a 1 m by 1 m specimen, the minimum frequency is about 10 kHz. A first hydrophone 14 is positioned on the opposite side of the sample 10 to measure the transmitted pressure. The ratio of the source pressure to the transmitted pressure expressed in decibels is the insertion loss of the material. A second hydrophone 16 is positioned on the same side of sample 10 as projector 12 to measure the reflected acoustic pressure.
Insertion loss can also be determined by measuring the motion of the test sample with either accelerometers or laser velocimeters and calculating the pressure field based on conservation of linear momentum. In the test setup shown here, a first laser velocimeter 18 is used to measure the acceleration and position of a first side 20 of sample 10. A second laser velocimeter 22 is used to measure the acceleration and position of a second side 24 of sample 10. Laser velocimeters 18, 22 are preferred because accelerometers must be positioned on sample 10 and might interfere with the measurements. The projector 12 angle θ relative to the test material can be changed so that the effects of acoustic energy at varying angles can be studied. Changing the excitation angle θ is equivalent to changing the excitation wavenumber. Thus, the two parameters that are typically varied during this test are frequency and wavenumber.
For underwater applications, the material is submerged in a fluid (normally water), and an underwater speaker or projector transmits energy at the material; however, a gaseous environment could be used. Because this test is only interested in acoustic attenuation of the material, the height and width of the test specimen are large compared to its thickness. In view of this, the test specimen should have a thickness between 10 mm and 100 mm. This prevents acoustic energy from moving around the specimen and contaminating the transmitted pressure field and interacting with the opposite side to the test specimen. The test is also dependent on the environment where it is conducted. Small test tanks prevent low frequency measurements due to reflection and reverberation of the acoustic energy. These are, however, practical limitations and do not enter into this theoretical analysis.